The notion of redundancy is well known as a safety means. A primary example is military aircraft, where redundant systems are prevalent. If the primary electrical system fails, there is another to take its place. If the primary fuel distribution system fails, there is another to take its place. The theory being that the probability of two systems failing simultaneously is far less than the probability of a single system failing.
Automotive breaking systems provide an example of redundant systems in the consumer market. Many cars today actually have dual braking systems. The redundant systems run from the same master cylinder to save cost, but utilize two distinct control systems to stop the wheels. If either system fails, the other takes over so that the car may be stopped safely.
Likewise, in the electronics industry, engineers and designers building protection circuits for safety applications have used redundant components and circuits for improved safety reliability. In such a redundant system, if a defect occurs in a particular protection circuit, a redundant circuit may still provide the required level of safety protection. As stated, the probability that two redundant circuits will fail simultaneously is much smaller than the probability that a single defect may occur in a single circuit. Thus, redundant circuits provide additional safety and security in safety protection circuits.
An example of an electronic component employing redundant technology is the Metal Oxide Silicon Field Effect Transistor (MOSFET). MOSFETs come in many shapes and sizes, depending upon the power that the MOSFET is designed to handle. Generally speaking, the larger the transistor, the more power that can be dissipated. Large transistors capable of handling more that 1 watt of power dissipation are colloquially known as "power transistors".
A common misconception with power transistors is the idea that they are simply large. People often think of a 1 watt transistor as having physical characteristics twice that of a 1/2 watt transistor. In reality, this is not so. Due to lower manufacturing costs, in a MOSFET cell matrix structure, the power MOSFET is actually manufactured as numerous (thousands is typical) transistors connected in parallel by metalization runners on the integrated circuit silicon. The effect of these numerous transistor cells operating in parallel is to achieve a high current handling capability. Each cell of the structure shares a tiny amount of the total current.
An example of this structure is shown in FIG. 1. This structure employs a plurality of MOSFET transistors 8 (although only four are shown, it will be understood that many transistors are typically employed), each connected to a common drain 2, a common source 6 and a common gate 4. Thus, if the common gate 4 is asserted, current will flow from the drain 2 to the source 6 (or vice versa, depending on whether n-channel or p-channel MOSFET's are used), with only a fraction of the total current flowing through each individual MOSFET 8.
The redundant transistor system works well on silicon because the tiny transistors are inexpensive to manufacture. When an application engineer builds products like rechargeable batteries, however, he must design in duplicate parts to achieve redundancy. For example, if a rechargeable battery designer uses a transistor as a voltage regulator, to achieve redundancy he must use two discrete transistors. Each transistor occupies its own package. While this type of redundancy is effective, it is expensive. It also requires more circuit board space to realize a safe circuit.
There is therefore a need for an improved semiconductor safety device.